Method for the preparation of pure elemental phosphorus



United States Patent METHOD FOR THE PREPARATION OF PURE ELEMENTAL PHOSPHORUS Bernard A. Gruber, Dayton, Ohio, assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Dec. 30, 1957, Ser. No. 705,829

14 Claims. (Cl. 204-60) This invention relates to a method for producing elemental phosphorus in a very pure form.

It is an object of the invention to prepare phosphorus which is free from the conventional impurities, such as compounds and elements of group 7 of the periodic table. It is also an object of the invention to prepare elemental phosphorus by the electrolysis of phosphates. Other objects of the invention are set forth in the description below.

It has been desired for many years to obtain elemental phosphorus in a very pure form, particularly in the preparation of semi-conductor components such as boron i phosphide and indium phosphide.

However, the order of purity which is necessary for such purposes requires that foreign elements and their compounds particularly the group VI and group VII elements, such as arsenic, antimony, bismuth, sulfur, selenium, tellurium, aluminum, gadolinium and chlorine be present in less than a fraction of one part per million. The use of conventional refining methods for the phosphorous have not been effective in obtaining such pure forms.

It has now been found that exceedingly pure elemental phosphorus may be obtained by the electrolysis of various phosphorus salts such as phosphates and phosphites, including the polyphosphates as the cell electrolyte. A preferred compound in this relationship is sodium metaphosphate, NaPO although the various phosphorous salts, preferably tetrasodium pyrophosphate or sodium tripolyphosphate may similarly be employed as the starting material. Other potassium and sodium compounds of phosphorus of particular utility are the di-alkali and tri-alkali phosphates, the hexametaphosphates and the various ortho-, metaand poly-phosphates which have a molar Na:P ratio of at least one. In another embodiment of the invention, the phosphate is employed in combination with a boron oxide, a boric acid or an alkali or alkaline earth borate (e.g., sodium tetraborate or calcium metaborate), the preferred compound being a boron oxide, the relative molar proportion of the boron compound with respect to the phosphorus compound being from 0.001 to 2, a preferred range being from 0.01 to 1.

' An example of a specific combination which has been found to be of utility is 16 moles of the sodium metaphosphate with one mole of boric oxide.

Another cell electrolyte which may advantageously be employed in the present process is boron phosphate, BPO Boron phosphate is known to have a high melting point, but this compound may readily be used in the present process by dissolving the boron phosphate in a suitable solvent compound such as sodium fluoride or a sodium phosphate. The potassium compounds may similarly be used.

'As an adjunct material in order to increase the fluidity of the cell electrolyte, and also to increase the conductivity thereof, additional salts such as alkali halides, for example, sodium fluoride and potassium chloride may also be present. When such additive materials are employed they may be present in the relative proportion of from 5 to mole percent relative to the cell electrolyte mixture, a preferred range being from 20 to 50 mole percent.

The present process is conducted in an electrolytic cell having an anode and a cathode. The vessel itself may be utilized as either the anode or cathode if desired. The anode is made of a form of carbon such as graphite, including the conventional graphite electrodes prepared from petroleum carbon and bonded with pitch. The cathode may also be made of graphite or a metal such as iron or other ferrous alloys, for example, the stainless steels, as well as nickel or molybdenum.

The electrolytic process of the present invention may be conducted without the necessity for employing a diaphragm in the cell. However, it may be desirable, particularly at high rates of production to introduce a diaphragm which prevents mixing of the anolyte and catholyte. The diaphragm is made from a porous form of a ceramic material such as alumina. The present process may also make use of a partition in the vapor phase to prevent mixing of the gaseous anode and cathode products. Such a partition or separator is usually made of a non-porous chemically resistant material such as graphite, alumina, silica or other ceramic.

The present electrolytic process employing a fluid melt is conducted at a temperature in the range of from 600 C. to 1,000 C., a preferred temperature rang being from 800 C. to 900 C. The pressure in the electrolytic cell is normally atmospheric, although it is possible to operate with moderate degrees of pressure and vacuum as may be desired to control the flow of materials leaving the electrolytic cell.

The electrical conditions employed for the production of elemental phosphorus in the above-described system include a voltage range of from 1 to 10 volts as the cell voltage, a preferred range being from 1.5 to 5 volts. The current density employed in thisprocess is from to 5,000 amperes per square foot. A preferred range for the current density is from 700 to 3,000 amperes per square foot.

The mechanism of the present process has not been completely elucidated, although it has clearly been demonstrated that gaseous elemental phosphorus is produced directly at the electrode and is vaporized from the cell. The oxygen of the phosphate ultimately leaves the system from the carbon electrode as oxides of carbon. The phosphorus may be condensed directly in a condenser system and be collected by conventional means. Inasmuch as the phosphorus leaves the cell in gaseous form at a relatively high temperature, such as about 800 C., it is likewise feasible to oxidize such product phosphorus directly by adding air or oxygen to the gas stream with the resultant production of phosphorus pentoxide, which is then condensed and collected as the ultimate product. In this relationship, it is also feasible to introduce the air or oxygen directly to the electrolytic cell to bring about the oxidation of the phosphorus as formed. On the other hand, if it is desired to avoid any oxidative elfects of the phosphorus being produced, the cell may be provided with an inert gas stream such as nitrogen or argon, which thus avoids any introduction of foreign reactive gases into the electrolytic cell or the contamination of the elemental phosphorus by undesired elements and compounds including dissolved gases. The present process also makes it possible to produce anhydrous phosphorus of an unusual degree of purity, since the electrolytic process is conducted under anhydrous conditions and permits the use of a collection system blanketed with an inert gas such as argon. In the instant process, an electrolytic reaction is clearly involved since it has been found that the production of phosphorus does not begin in the heated cell until the electric current begins to pass.

phosphorus may be withdrawn as a gaseous product which is continuously being evolved at theelectrode. The

charge nia'ter'ialjeither' air alk'ali phosphate; boron 'p h'osphate, or a mixture of an alkalipli'osphate" arid a"bora te, a; boric acid or boron, oxide, is admitted directly to the rriolteni electrolyte mass; Such introduction of said mate-V rialsj'm'ay accomplishedfhy: pumpingfiri a" fluidimelt; or

may be accomplished" tpraagh are use or a reparation;

section of the eellfiii whi'chthe raw m'aterials' are first chargedfland permitted 'to evolve stear'h'andi other gases;

before being admittedto" the electrolytic section of are cell."

of the present invention? f r Example] The production ofelei'hen'tal phosphorus from a les-j p cell charged refine: produetioii of. elemental 30 phoruswas prepared by mixing 122 g.; of sodium acids nh a e h rf tslafi firstth o de; J electrolytic cell was heated to a, temperature of 800 C.

atlwhich point the fluidity otthe meltiwas increased by. adding about 10 g. of potassium chloride. The electrodes consisted of,a one-half inchgraphite rod as, theianode located in thefcenter of thegraphi e crucible whichserved as the'cathodel ,The electrolysis began at a cell voltage of volts ,and with an amperage of 15 amperes It was found that-during the steady state operation of the; cell theiyoltage couldbe reduced to 2.6 volts with the cell operating vat 5 'amperes. U During the electrolysis, gaseous elementalphosphorus was evolved at the anode and, removed from the cell. The phosphorus thus produced is substantially free of group VI and VII co ntaminants. At the conclusion of the run, the electrolyte was withdrawn The solublephosphatesand chlorides were leached out to'lcave an insoluble component which was found to correspond'to boron phosphate by X-ray diffraction.

Example 3 A cellelectrolytejwas prepareduby mixing 275 g ofv sodium" acidflphosphat' mbneh'yd'rate, 21g. of sodium fluoride, an 8 7 g, of boric acid. The electrolysis was conducte e apparatus describedlin. Example 1, the initial conditions rbe'ing' 2l'3 volts and 10: amper'esQ The electrolysisfwas'" found to, run smoothly with the cell volt- '0.'9"tol 2.7 volts: and with the amperage .y g fro rnfllto 25 amper'es. It vwas tound that phosphorus n gaseousfform was evolved in copious qi' ntitis during' he' perationof the cell and could read, Y bewitnn ,a n for collection.

What is claimed isi Trauma- "shapes illustrate speifieeimiaainiius" alkali phosphate, boron phosphate, and mixtures of alkali phosphates with at least one eompound selected from the group consisting of boron oxides, boric acid, alkali borates, and alkaline earth borates, the said electrolysis being conducted at a temperature in the range of from 600 C. to 1,000 C., and with a cell voltage of from 1 to 10 volts and a current density of from 100 to 5,000 amperes per square foot in whichlmixturestth'e, relative molar proportion of the boron compound relative to the phosphate compound isl in the range of from"0.0l01to' 2 and, evolvingphos phoriisand collectirig the same. c u I;

3. Process for the production of elemental phosphorus which comprises electrolyziiig a niolt'en bath-consisting of an electrolyteselected from the, group consisting of an alkali phosphate, boron'pho's phate; and mixtures of alkali phosphates with at least one compound selected from the group consisting of boron oxides,boric acid, alkali borates, and alkaline earth borates, the said electrolysis .being conducted at a temperature in the range of from 800 C. to 900 C. and with a cell voltage of from 1.5 to 5 volts and with a current density of from 700 to 3,000 amperes per square foot in which mixtures the relative molar proportion of the boron compound relative to the phosphate compound is inthe range of from 0.001 to 2 and, evolving phosphorus and collecting the same.

A. Process for the production of elementalphosphorus which comprises electrolyzing a molten bath consisting ofa mixture of a sodium phosphate, with at least one compound selectedvfrom'. the group consisting of boron oxides, boric acid; alkali borates and alkaline earth borates 'in which mixturesithe'relative molar proportion ofIth e boron compound relative to the phosphate compound is in therange of from 0.001 to 2 and, evolving phosphorusand collecting the same.

5'. Process for the production of elemental phosphorus which comprises electrolyzing a molten bath consisting of a mixtureof a sodium phosphate and a boron oxide in which mixture the relative molar proportion of the boron oxide relative to the sodium phosphate is in the rangefof from 0.001 to 2; and, evolving phosphorus and collecting the same. a

6. Process'forthe production of elemental phosphorus Whichcomprises electrolyzing a molten bath consisting of a sodium phosphate and a boron oxide in which mixture the relativemolar proportion of the boron oxide relative to thesodiuin phosphate is in the range of from 0.001 to 2; and, the said electrolysis being conducted at a temper'ature in the range of from 600 C. to 1,000 C., and

with a cell voltage of 'from 1 'to' 10 volts and a current density of'from 100 to 5,000 amperes 'per square foot,'

8.r,P'rocessfor the production of elemental phosphorus which comprises rele ctrolyzing a molten bath consisting of a sodium phosphate incombination with a boron oxide,

the relativemolarproportion or the said boron oxide relative to ithe said phosphate compound ".beingl the range H of from0.00l to}; and as a modifying agent thereinan alkali halidelthe said modifying agent being present fin the proportion ofvfrom 5 mole percent .to 'mole percen't' relativevto theQ'aforesaid electrolyte, evolviugphosphorus sn1ee11tingrhejsame; 7

qhtwa sese less; of a sodium, phosphate in combination with the relative molar "pro ertiesbfthesad bdiea anaemic tive to the said phosphate compound being in the range of from 0.01 to 1, and as a modifying agent therein an alkali halide, the said modifying a gent being present in the proportion of from 40 mole percent to 90 mole percent by weight relative to the weight of the aforesaid electrolyte, the said electrolysis being conducted at a temperature in the range of from 600 C. to 1,000 O, and with a cell voltage of from 1 to volts and a current density of from 100 to 5,000 amperes per square foot, evolving phosphorus and collecting the same.

10. Method for the preparation of elemental phosphorus which comprises electrolyzing molten boron phosphate, in a cell, evolving phosphorus from the said molten boron phosphate and collecting the said phosphorus.

11. Process for the production of elemental phosphorus which comprises electrolyzing molten boron phosphate and as a modifying agent therein an alkali halide, the said modifying agent being present in the proportion of from 5 mole percent to 80 mole percent by weight relative to the weight of the aforesaid electrolyte, the said electrolysis being conducted at a temperature in the range of from 800 C. to 900 C., and with a cell voltage of from 1.5 to 5 volts and with a current density of from 700 to 3,000 amperes per square foot, evolving phosphorus and collecting the same.

12. Method for the preparation of elemental phosphorus which comprises electrolyzing a molten electrolyte consisting of NaPO in combination with a boron compound selected from the class consisting of boron oxides, boric acid, alkali borates, and an alkaline earth borate, the relative proportion of the said boron compound and the said phosphate compound being in the range of from 0.001 to 2, evolving phosphorus and collecting the same.

13. Method for the preparation of elemental phos phorus which comprises electrolyzing a molten electrolyte consisting of NaPO in combination witlira boron compound selected from the class consisting of boron oxides, boric acid, an alkali borate, and an alkaline earth borate, the relative molar proportion of the said boron compound with respect to the phosphate compound being in the range of from 0.01 to l, the said electrolysis being conducted at a temperature in the range of from 600 C. to 1,000 C., and with a cell voltage of from 1 to 10 volts and a current density of from to 5,000 amperes per square foot, evolving phosphorus and collecting the same.

14. Method for the preparation of elemental phosphorus which comprises electrolyzing a molten electrolyte consisting of NaPO in combination with a boron compound selected from the class consisting of boron oxides, boric acid, an alkali borate, and an alkaline earth borate, the relative molar proportion of the said boron compound with respect to the said phosphate compound being in the range of from 0.01 to 1, the said electrolysis being conducted at a temperature in the range of from 800 C. to 900 C., and with a cell voltage of from 0.5 to 5 volts and with a current density of from 700 to 3,000 amperes per square foot, evolving phosphorus and collecting the same.

References Cited in the file of this patent UNITED STATES PATENTS 2,810,683 Ellis Oct, 22, 1957 FOREIGN PATENTS 658,521 France Jan. 26, 1929 

1. PROCESS FOR THE PRODUCTION OF ELEMENTAL PHOSPHORUS WHICH COMPRISES ELECTROLYZING A MOLTEN BATH CONSISTING OF AN ELECTROLYTE SELECTED FROM THE GROUP CONSISTING OF AN ALKALI PHOSPHATE, BORON PHOSPHATE, AND MIXTURES OF ALKALI PHOSPHATES WITH AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF BORON OXIDES, BORIC ACID, ALKALI BORATES AND ALKALINE EARTH BORATES IN WHICH MIXTURES THE RELATIVE MOLAR PROPORTION OF THE BORON COMPOUND RELATIVE TO THE PHOSPHATE COMPOUND IS IN THE RANGE OF FROM 0.001 TO 2 AND EVOLVING PHOSPHORUS AND COLLECTING THE SAME. 